Mechanical throttle control systems use airflow as the primary control parameter in controlling engine or powertrain output. The airflow is controlled by a throttle valve in the intake which is mechanically linked to a throttle pedal. As such, many powertrain control parameters, including automatic transmission shift scheduling, were traditionally based on, or indexed by, the throttle valve position.
Electronic airflow control systems, such as variable cam timing systems and electronic throttle control systems, replace the traditional mechanical throttle cable system with an "electronic linkage" provided by sensors and actuators in communication with an electronic controller. This increases the control authority of the electronic controller and allows the throttle valve to be controlled independently of the pedal position. As such, the throttle valve position is no longer necessarily indicative of the requested or desired powertrain output.